[PATCHv4 11/15] clk: ti: clockdomain: add clock provider support to clockdomains

[Tero Kristo]

On 03/12/16 02:18, Tony Lindgren wrote:
> * Michael Turquette <mturquette at baylibre.com> [161202 15:52]:
>> Quoting Tony Lindgren (2016-12-02 15:12:40)
>>> * Michael Turquette <mturquette at baylibre.com> [161202 14:34]:
>>>> Quoting Tony Lindgren (2016-10-28 16:54:48)
>>>>> * Stephen Boyd <sboyd at codeaurora.org> [161028 16:37]:
>>>>>> On 10/28, Tony Lindgren wrote:
>>>>>>> * Tero Kristo <t-kristo at ti.com> [161028 00:43]:
>>>>>>>> On 28/10/16 03:50, Stephen Boyd wrote:
>>>>>>>>> I suppose a PRCM is
>>>>>>>>> like an MFD that has clocks and resets under it? On other
>>>>>>>>> platforms we've combined that all into one node and just had
>>>>>>>>> #clock-cells and #reset-cells in that node. Is there any reason
>>>>>>>>> we can't do that here?
>>>>>>>>
>>>>>>>> For OMAPs, there are typically multiple instances of the PRCM around; OMAP4
>>>>>>>> for example has:
>>>>>>>>
>>>>>>>> cm1 @ 0x4a004000 (clocks + clockdomains)
>>>>>>>> cm2 @ 0x4a008000 (clocks + clockdomains)
>>>>>>>> prm @ 0x4a306000 (few clocks + resets + power state handling)
>>>>>>>> scrm @ 0x4a30a000 (few external clocks + plenty of misc stuff)
>>>>>>>>
>>>>>>>> These instances are also under different power/voltage domains which means
>>>>>>>> their PM behavior is different.
>>>>>>>>
>>>>>>>> The idea behind having a clockdomain as a provider was mostly to have the
>>>>>>>> topology visible : prcm-instance -> clockdomain -> clocks
>>>>>>>
>>>>>>> Yeah that's needed to get the interconnect hierarchy right for
>>>>>>> genpd :)
>>>>>>>
>>>>>>>> ... but basically I think it would be possible to drop the clockdomain
>>>>>>>> representation and just mark the prcm-instance as a clock provider. Tony,
>>>>>>>> any thoughts on that?
>>>>>>>
>>>>>>> No let's not drop the clockdomains as those will be needed when we
>>>>>>> move things into proper hierarchy within the interconnect instances.
>>>>>>> This will then help with getting things right with genpd.
>>>>>>>
>>>>>>> In the long run we just want to specify clockdomain and the offset of
>>>>>>> the clock instance within the clockdomain in the dts files.
>>>>>>>
>>>>>>
>>>>>> Sorry, I have very little idea how OMAP hardware works. Do you
>>>>>> mean that you will have different nodes for each clockdomain so
>>>>>> that genpd can map 1:1 to the node in dts? But in hardware
>>>>>> there's a prcm that allows us to control many clock domains
>>>>>> through register read/writes? How is the interconnect involved?
>>>>>
>>>>> There are multiple clockdomains, at least one for each interconnect
>>>>> instance. Once a clockdomain is idle, the related interconnect can
>>>>> idle too. So yeah genpd pretty much maps 1:1 with the clockdomains.
>>>>>
>>>>> There's more info in for example omap4 TRM section "3.4.1 Device
>>>>> Power-Management Layout" that shows the voltage/power/clock domains.
>>>>> The interconnect instances are mostly named there too looking at
>>>>> the L4/L3 naming.
>>>>
>>>> I'm confused on two points:
>>>>
>>>> 1) why are the clkdm's acting as clock providers? I've always hated the
>>>> name "clock domain" since those bits are for managing module state, not
>>>> clock state. The PRM, CM1 and CM2 provide the clocks, not the
>>>> clockdomains.
>>>
>>> The clock domains have multiple clock inputs that are routed to multiple
>>> child clocks. So it is a clock :)
>>>
>>> See for example omap4430 TRM "3.6.4 CD_WKUP Clock Domain" on page
>>> 393 in my revision here.
>>>
>>> On that page "Figure 3-48" shows CD_WKUP with the four input clocks.
>>> And then "Table 3-84. CD_WKUP Control and Status Parameters" shows
>>> the CD_WKUP clock domain specific registers. These registers show
>>> the status, I think they are all read-only registers. Then CD_WKUP
>>> has multiple child clocks with configurable registers.
>>>
>>> From hardware register point of view, each clock domain has:
>>>
>>> - Read-only clockdomain status registers in the beginning of
>>>   the address space
>>>
>>> - Multiple similar clock instances register instances each
>>>   mapping to a specific interconnect target module
>>>
>>> These are documented in "3.11.16.1 WKUP_CM Register Summary".
>>
>> Oh, this is because you are treating the MODULEMODE bits like gate
>> clocks. I never really figured out if this was the best way to model
>> those bits since they do more than control a line toggling at a rate.
>> For instance this bit will affect the master/slave IDLE protocol between
>> the module and the PRCM.
>
> Yes seems like there is some negotiation going on there with the
> target module. But from practical point of view the CLKCTRL
> register is the gate for a module functional clock.

There's some confusion on this, clockdomain is effectively a collection 
of clocks, and can be used to force control that collection if needed. 
Chapter "3.1.1.1.3 Clock Domain" in some OMAP4 TRM shows the 
relationship neatly.

>
>>> From hardware point of view, we ideally want to map interconnect
>>> target modules to the clock instance offset from the clock domain
>>> for that interconnect segment. For example gptimer1 clocks would
>>> be just:
>>>
>>> clocks = <&cd_wkup 0x40>;
>>>
>>>> 2) why aren't the clock domains modeled as genpds with their associated
>>>> devices attached to them? Note that it is possible to "nest" genpd
>>>> objects. This would also allow for the "Clockdomain Dependency"
>>>> relationships to be properly modeled (see section 3.1.1.1.7 Clock Domain
>>>> Dependency in the OMAP4 TRM).
>>>
>>> Clock domains only route clocks to child clocks. Power domains
>>> are different registers. The power domains map roughly to
>>> interconnect instances, there we have registers to disable the
>>> whole interconnect when idle.
>>
>> I'm not talking about power islands at all, but the genpd object in
>> Linux. For instance, if we treat each clock domain like a clock
>> provider, how could the functional dependency between clkdm_A and
>> clkdm_B be asserted?
>
> To me it seems that some output of a clockdomain is just a input
> of another clockdomain? So it's just the usual parent child
> relationship once we treat a clockdomain just as a clock. Tero
> probably has some input here.

A clockdomain should be modelled as a genpd, that I agree. However, it 
doesn't prevent it from being a clock provider also, or does it?

>> There is certainly no API for that in the clock framework, but for genpd
>> your runtime_pm_get() callback for clkdm_A could call runtime_pm_get
>> against clkdm_B, which would satisfy the requirement. See section
>> 3.1.1.1.7 Clock Domain Dependency in the OMAP4 TRM, version AB.

For static dependencies the apis genpd_add/remove_subdomain could 
probably be used.

> To me it seems the API is just clk_get() :) Do you have some
> specific example we can use to check? My guess is that the
> TRM "Clock Domain Dependency" is just the usual parent child
> relationship between clocks that are the clockdomains..
>
> If there is something more magical there certainly that should
> be considered though.

The hwmods could be transformed to individual genpds also I guess. On DT 
level though, we would still need a clock pointer to the main clock and 
a genpd pointer in addition to that.

Tony, any thoughts on that? Would this break up the plans for the 
interconnect completely?

-Tero